This invention relates to flow control valves, and more particularly, to a flow control valve which includes guide means to minimize fluid flow forces tending to hold the spool open.
It is common for fluid control valves to include annularly notched valve spools which are reciprocable in cylidrical bores of a housing. A plurality of fluid passages are disposed in the housing in intersecting relation with such bores in order to direct fluid from a source to various implements or the like for operation thereof. As fluid enters such a bore, it impinges on the various surfaces of the valve spool when the valve is metering or in its open position and generates axial thrust forces on the spool which tend to hold the spool open. This problem is particularly noticeable in high pressure circuits using in the order of 2,000 psi or more and having relatively high flow rates.
Such flow-generated axial force means that the force required to move the spool towards the open position is less than the force required to close it, which is a very undesirable state. In manually positioned valve spool systems, such unequal forces are not only distracting to the operator of the valve, but could add a dimension of complexity that could be dangerous. In automatic or servo-operated spool positioning systems, as well as the manually operated type, the different force requirements could deterimentally affect the rate of response of the spool in each direction of movement thereof, or make it necessary to increase the forces associated therewith to overcome the dynamic fluid biasing forces, such as by adding heavier spool reaction springs or the like, which would be undesirable from a performance standpoint.
Such flow-generated fluid forces have been recognized as a significant problem heretofore, and reference is made to U.S. Pat. Nos. 2,971,536 to Junck et al and 3,198,212 to Junck et al, both assigned to the Assignee of the present invention. However, adding metering slots or tapered surfaces to the spool on the downstream side of the metering path in an attempt to impart axially balancing thrust forces does not always produce the desired results. In addition, the relatively sophisticated nature of the slot dimensions and tapers makes it necessary to resort to extensive testing programs before successful results can be achieved.
In addition, as will later be described, the use of a cross-drilled fluid inlet passage rather than a complete fluid inlet annulus in the housing adds to the difficulty of directing flow adequately without contributing significantly to side loads on the spool.
Reference is also made to U.S. Pat. No. 3,620,230 to Stahle, which includes an axial thrust generating barrier in combination with the housing thereof. Such barrier, it will be seen, would interfere with axial movement of the spool thereof after limited travel of said spool, and would also interfere with the free removal of the valve spool for servicing thereof. In addition, it is to be appreciated that constructional changes for the purpose of flow control in such a valve should preferably be made to the spool itself, rather than in combination with the housing, as such modifications are substantially less costly than modifying the internal passages within a housing.
Of more general interest in this area are U.S. Pat. No. 2,747,612 to Lee U.S. Pat. No. 2,812,775 to Hodgson, and U.S. Pat. No. 3,009,480 to Miller.